1. Field of the Invention
The present invention relates to an OLED (organic light emitting device) panel obtained by forming an OLED on a substrate and sealing the OLED between the substrate and a cover member. The invention also relates to an OLED module in which an IC including a controller, or the like, is mounted to the OLED panel. In this specification, light emitting device is the generic term for the OLED panel and for the OLED module. Electronic devices using the light emitting device are also included in the present invention.
2. Description of the Related Art
Being self-luminous, OLEDs eliminate the need for a backlight that is necessary in liquid crystal display devices (LCDs) and thus make it easy to manufacture thinner devices. Also, the self-luminous OLEDs are high in visibility and have no limitation in terms of viewing angle. These are the reasons for the attention that light emitting devices using the OLEDs are receiving in recent years as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of the light emission.
In this specification, all the layers that are provided between an anode and a cathode together make an organic light emitting layer. Specifically, the organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
Hereinafter, a structure of a pixel in a general light emitting device will be described using FIG. 15.
In a pixel portion of a general light emitting device, a plurality of pixels 1000 are provided in a matrix shape. Each pixel 1000 includes at least one signal line 1001, at least one scan line 1002, and at least one power source line 1003.
Also, the pixel 1000 includes a switching TFT 1004, a driver TFT 1005, an OLED 1006, and a storage capacitor 1007.
The gate electrode of the switching TFT 1004 is connected with the scan line 1002. With respect to the source region and the drain region of the switching TFT 1004, one is connected with the signal line 1001 and the other is connected with the gate electrode of the driver TFT 1005.
The storage capacitor 1007 is formed between the gate electrode of the driver TFT 1005 and the power source line 1003. The storage capacitor 1007 is provided to hold a gate voltage (difference of potential between the gate electrode and the source region) of the driver TFT 1005 in the case when the switching TFT 1004 is in a non-select state (off state).
Also, with respect to the source region and the drain region of the driver TFT 1005, one is connected with the power source line 1003 and the other is connected with the OLED 1006.
The OLED 1006 is composed of an anode, a cathode, and an organic light emitting layer provided between the anode and cathode. When the anode is connected with the source region or the drain region of the driver TFT 1005, the anode is called a pixel electrode and the cathode is called a counter electrode. On the other hand, when the cathode is connected with the source region or the drain region of the driver TFT 1005, the cathode is called a pixel electrode and the anode is called a counter electrode.
A potential (counter potential) is applied to the counter electrode of the OLED 1006 by a power source provided outside an OLED panel. Also, a potential (power source potential) is applied to the power source line 1003 by the power source provided outside the OLED panel.
Next, an operation of the pixel 1000 shown in FIG. 15 will be described.
When the scan line 1002 is selected in response to a selection signal inputted to the scan line 1002, the switching TFT 1004 in which the gate electrode is connected with the scan line 1002 becomes an on state. Note that, in this specification, the selection of the scan line means that all TFTs in which the gate electrodes are connected with the scan line are tuned on.
Then, a video signal having image information inputted to the signal line 1001 is inputted to the gate electrode of the driver TFT 1005 through the switching TFT 1004 which is turned on.
In accordance with a potential of the video signal inputted to the gate electrode, a gate voltage of the driver TFT 1005 is determined. A current of a value corresponding to the gate voltage flows into the channel forming region of the driver TFT 1005. The current flowing into the channel forming region of the driver TFT 1005 flows into the OLED 1006.
When the current flows into the OLED 1006, the OLED 1006 emits light. When the above operation is performed for all pixels, an image is thus displayed on the display portion.
Now, the driver TFT is ideal to be in a normally off state. For example, the following configuration is ideal in the case of a p-channel TFT. That is, when a gate voltage (potential between the source region and the drain region) is larger than a threshold value, a drain current does not flow. On the other hand, only when the gate voltage becomes smaller than the threshold value, the drain current starts to flow. Also, the following configuration is ideal in the case of an n-channel TFT. That is, when the gate voltage is smaller than the threshold value, the drain current does not flow. On the other hand, only when the gate voltage becomes larger than the threshold value, the drain current starts to flow. Note that, in this specification, the increase in the gate voltage means that the gate voltage is changed in a positive direction and the decrease in the gate voltage means that the gate voltage is changed in a negative direction.
The threshold voltage is ideal to be a negative value in the case of a p-channel TFT. On the other hand, the threshold voltage is ideal to be a positive value in the case of an n-channel TFT.
However, actually, the threshold voltage of a TFT is shifted somewhat according to a manufacturing step. When the threshold voltage is shifted, there is the case where the driver TFT which should become an off state is turned on. When the driver TFT which should become an off state is turned on, the drain current flows into the channel forming region of the driver TFT and then the OLED emits light even when light emission is not required. This becomes a cause of reduced contrast or disturbed display image.
Also, there is a case where a current flowing in an off state (off current) becomes large, depending on a characteristic of a TFT. When the off current of the driver TFT is large, such a current flows into the OLED. Thus, the OLED emits light even when light emission is not required.
In order to reduce an off current, there are proposed a method of increasing a channel length of the driver TFT and a method of increasing the number of gate electrodes to obtain a multi-gate structure. However, in either of the methods, there is a limitation regarding reduction in the off current.